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<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/linearLayout1"
android:layout_marginLeft="10pt"
android:layout_marginRight="10pt"
android:layout_marginTop="3pt">
<EditText
android:layout_weight="1"
android:layout_height="wrap_content"
android:layout_marginRight="5pt"
android:id="@+id/etNum1"
android:layout_width="match_parent"
android:inputType="numberDecimal">
</EditText>
<EditText
android:layout_height="wrap_content"
android:layout_weight="1"
android:layout_marginLeft="5pt"
android:id="@+id/etNum2"
android:layout_width="match_parent"
android:inputType="numberDecimal">
</EditText>
</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/linearLayout2"
android:layout_marginTop="3pt"
android:layout_marginLeft="5pt"
android:layout_marginRight="5pt">
<Button
android:layout_height="wrap_content"
android:layout_width="match_parent"
android:layout_weight="1"
android:text="+"
android:textSize="8pt"
android:id="@+id/btnAdd">
</Button>
<Button
android:layout_height="wrap_content"
android:layout_width="match_parent"
android:layout_weight="1"
android:text="-"
android:textSize="8pt"
android:id="@+id/btnSub">
</Button>
<Button
android:layout_height="wrap_content"
android:layout_width="match_parent"
android:layout_weight="1"
android:text="*"
android:textSize="8pt"
android:id="@+id/btnMult">
</Button>
<Button
android:layout_height="wrap_content"
android:layout_width="match_parent"
android:layout_weight="1"
android:text="/"
android:textSize="8pt"
android:id="@+id/btnDiv">
</Button>
</LinearLayout>
<TextView
android:layout_height="wrap_content"
android:layout_width="match_parent"
android:layout_marginLeft="5pt"
android:layout_marginRight="5pt"
android:textSize="12pt"
android:layout_marginTop="3pt"
android:id="@+id/tvResult"
android:gravity="center_horizontal">
</TextView>
</LinearLayout>

package com.example.calculator;

import android.os.Bundle;
import android.app.Activity;
import android.text.TextUtils;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends Activity {
	
	  EditText etNum1;
	  EditText etNum2;

	  Button btnAdd;
	  Button btnSub;
	  Button btnMult;
	  Button btnDiv;

	  TextView tvResult;

	  String oper = "";
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		// find the elements
	    etNum1 = (EditText) findViewById(R.id.etNum1);
	    etNum2 = (EditText) findViewById(R.id.etNum2);

	    btnAdd = (Button) findViewById(R.id.btnAdd);
	    btnSub = (Button) findViewById(R.id.btnSub);
	    btnMult = (Button) findViewById(R.id.btnMult);
	    btnDiv = (Button) findViewById(R.id.btnDiv);

	    tvResult = (TextView) findViewById(R.id.tvResult);

	    // set a listener
	    btnAdd.setOnClickListener((OnClickListener) this);
	    btnSub.setOnClickListener((OnClickListener) this);
	    btnMult.setOnClickListener((OnClickListener) this);
	    btnDiv.setOnClickListener((OnClickListener) this);
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}
	
	
	  public void onClick(View v) {
		  
	    // TODO Auto-generated method stub
	    float num1 = 0;
	    float num2 = 0;
	    float result = 0;

	    // check if the fields are empty
	    if (TextUtils.isEmpty(etNum1.getText().toString())
	        || TextUtils.isEmpty(etNum2.getText().toString())) {
	      return;
	    }

	    // read EditText and fill variables with numbers
	    num1 = Float.parseFloat(etNum1.getText().toString());
	    num2 = Float.parseFloat(etNum2.getText().toString());

	    // defines the button that has been clicked and performs the corresponding operation
	    // write operation into oper, we will use it later for output
	    switch (v.getId()) {
	    case R.id.btnAdd:
	      oper = "+";
	      result = num1 + num2;
	      break;
	    case R.id.btnSub:
	      oper = "-";
	      result = num1 - num2;
	      break;
	    case R.id.btnMult:
	      oper = "*";
	      result = num1 * num2;
	      break;
	    case R.id.btnDiv:
	      oper = "/";
	      result = num1 / num2;
	      break;
	    default:
	      break;
	    }

	    // form the output line
	    tvResult.setText(num1 + " " + oper + " " + num2 + " = " + result);
	  
	  }
}